What Makes Something A Planet, According To An Astrophysicist?
“A dolphin may look like a fish, but it’s really a mammal. Similarly, the composition of an object is not the only factor in classifying it: its evolutionary history is inextricably related to its properties. Scientists will likely continue to argue over how to best classify all of these worlds, but it’s not just astronomers and planetary scientists who have a stake in this. In the quest to make organizational sense of the Universe, we have to confront it with the full suite of our knowledge.
Although many will disagree, moons, asteroids, Kuiper belt and Oort cloud objects are fascinating objects just as worthy of study as modern-day planets are. They may even be better candidates for life than many of the true planets are. But each object’s properties are inextricably related to the entirety of its formation history. When we try to classify the full suite of what we’re finding, we must not be misled by appearances alone.”
You’ve heard about the IAU’s definition, where in order to be a planet, you must pull yourself into hydrostatic equilibrium, orbit the Sun and nothing else, and gravitationally clear your orbit. You’ve also heard about the controversial new definition from geophysical/planetary science arguments, that planets should be based on their ability to pull themselves into a spheroidal shape alone.
Triton, Not Pluto or Eris, Is The Kuiper Belt’s Largest World
“The result, today, is that the largest and most massive body ever to form in the Kuiper belt — 20% larger than Pluto; 29% more massive than Eris — is now Neptune’s largest moon: Triton. Today, Triton makes up 99.5% of the mass orbiting Neptune, an enormous departure from all the other giant planet systems we know of. The only explanation for its properties, especially its bizarre and unique orbit, is that Triton is a captured Kuiper belt object.
We often talk about icy moons with subsurface oceans as candidate worlds for life. We imagine large, distant, icy bodies as planets or dwarf planets in their own right. Triton was born not as a moon of Neptune, but as the largest and most massive Kuiper belt object to survive. You don’t cease to exist when you move locations, and neither did Triton. It’s the original king of the Kuiper belt, and its true origin story is a cosmic mystery that deserves to be solved.”
In October of 1846, just months after Neptune was discovered, a large moon was discovered around it: Triton. Today, Triton is a supremely unusual moon for a number of reasons, but the largest is that it rotates in the wrong direction. While Neptune orbits the Sun counterclockwise and spins counterclockwise on its tilted axis, Triton orbits in the opposite direction. The only way this could have happened is if it were a captured object. And that’s exactly what it looks like: a captured object from the Kuiper belt!
Is Humanity Ignoring Our First Chance For A Mission To An Oort Cloud Object?
“In 2003, scientists discovered an object beyond Neptune that was unlike any other: Sedna. While there were larger dwarf planets beyond Neptune, and comets that would travel farther from the Sun, Sedna was unique for how far it always remained from the Sun. It always remained more than twice as distant from the Sun as Neptune was, and would achieve a maximum distance nearly 1,000 times as far as the Earth-Sun distance. And despite all that, it’s extremely large: perhaps 1,000 kilometers in diameter. It’s the first object we’ve ever found that might have originated from the Oort cloud. And we’ll only get two chances if we want to send a mission there: in 2033 and 2046. Right now, there isn’t even a proposed NASA mission looking at the possibility. If we do nothing, the opportunity will simply pass us by.”
Out beyond the eight planets of our Solar System, a large number of regions, all containing frozen objects, are theorized to exist. Innermost is the Kuiper belt, consisting of a wide variety of bodies, but all of which come quite close to Neptune’s orbit and feel its gravitational influence. Beyond that are the scattered disk objects: objects kicked by one of the gas giants out to greater distances. Beyond that are the detached objects, which have undergone multiple gravitational interactions and no longer come close to Neptune. And finally, there are the sednoids: objects that never come within double the Sun-Neptune distance of the Sun. There are only two known, and the first one, Sedna, is so large that it’s surely a dwarf planet. With an aphelion of approximately 1000 A.U., it may well have an origin in the inner Oort cloud, which is hitherto only theorized.
You Won’t Like The Consequences Of Making Pluto A Planet Again
“There are some out there who are desperate to save Pluto’s planetary status, and would be willing to open the floodgates and bestow planethood on every moon, asteroid, and ice ball out there that’s massive enough to be round. There are others who spend 100% of their time looking down at their feet on whatever world they’re considering when it comes to planethood, and to them, everything with enough mass will be a planet. But for the rest of us, where you are in the Universe is an inseparable part of what you are. Nothing in the Universe exists in a vacuum, and where you are determines a huge number of properties of you, regardless of whether you’re a planet, moon, asteroid, centaur, comet, Kuiper belt object, or Oort cloud object. If you want to ignore all of that — and proclaim, “round means planet” — then more power to you. But in planethood, as in most things, the full scientific story is far more interesting.”
When you say, “Pluto should be a planet,” what I hear is, “let’s ignore all of astronomy.” When you say, “we’re using a geophysical definition of a planet,” I hear, “we don’t believe in looking up.” And when you say, “we call ourselves planetary scientists, and so we get to decide what a planet is,” I hear, “we don’t care about the full suite of scientific evidence.” There is a long and interesting history to planets and planethood, and yes, the IAU definition is flawed. But does that mean, as Alan Stern and David Grinspoon contend, that we should call every object that can pull itself into a round shape a planet?
Interstellar Visitor ‘Oumuamua Was Shaped By Cosmic Particles
“We think of space as being an empty place, but the truth is that there are dust grains, particles, neutral atoms, ions, and cosmic rays zipping through the entirety of the galaxy, even when there are no stars. As an object moves through space, circling the galaxy at hundreds of kilometers per second (and moving relative to most other objects at tens of kilometers per second), it’s constantly bombarded by large numbers of small, fast-moving bits of matter. Just as water and sand will smooth out and erode pebbles and cobbles in the ocean here on our world, the cosmic equivalent — the interstellar medium — will have the same effect over extremely long timescales on ejected icy bodies.”
When scientists discovered ‘Oumuamua last year, they were surprised to find that it not only originated from outside our Solar System, but possessed bizarre properties we had never seen before. It was extremely elongated, tumbled irregularly, and had a never-before-seen composition: a carbon crust over an icy interior. Despite heating up to 550 °F (290
°C), it never developed a tail, a coma, or showed any ejecta. Many have proposed exotic or recent origins for this interstellar interloper, but in this case, simplicity rules: it may just be a cosmic pebble in the galactic sea. The interstellar medium is full of particles, and ‘Oumuamua, like most interstellar objects, should move at about 0.01% the speed of light through the galaxy. Over time, it should be worn down in exactly the fashion we see. As we discover more objects with an origin beyond our Solar System, we fully expect they’ll appear quite similar to this one.
Further analysis showed that these are driven by liquid water, not avalanches.
As the seasons change, water condenses and dissolves martian salts.
These then flow down the crater, as before-and-after images demonstrate.
If you think of Mars as a boring, reddish, desert world with dunes, sands, and craters, you’ve never seen it with the proper eyes. The Mars Reconnaissance Orbiter, equipped with its HiRISE camera, was launched more than a decade ago, and has covered the entire surface of the red planet, taking more than 50,000 images in enhanced color. It’s revealed countless features, shown us the insides of crater walls, viewed the martian bedrock, found impact craters, discovered evidence for liquid, flowing water, and even helped determine the origin of Mars’ moons. Yet to fully appreciate what we’ve learned, you simply have to see it for yourself.
Asteroid impacts are a major force in shaping planetary bodies over the course of their geological history. As such, they receive a great deal of attention and study, often in the form of simulations like the one above. This simulation shows an impact in the Orientale basin of the moon, and if it looks somewhat fluid-like, there’s good reason for that. Impacts like these carry enormous energy, about 97% of which is dissipated as heat. That means temperatures in impact zones can reach 2000 degrees Celsius. The rest of the energy goes into deforming the impacted material. In simulations, those materials – be they rock or exotic ices – are usually modeled as Bingham fluids, a type of non-Newtonian fluid that only deforms after a certain amount of force is applied. An everyday example of such a fluid is toothpaste, which won’t extrude from its tube until you squeeze it.
The fluid dynamical similarities run more than skin-deep, though. For decades, researchers looked for ways to connect asteroid impacts with smaller scale ones, like solid impacts on granular materials or liquid-on-liquid impacts. Recently, though, a group found that liquid-on-granular impacts scale exactly the way that asteroid impacts do. Even the morphology of the craters mirror one another. The reason this works has to do with that energy dissipation mentioned above. As with asteroid impacts, most of the energy from a liquid drop impacting a granular material goes into something other than deforming the crater region. Instead of heat, the mechanism for dissipation here is the drop’s deformation. The results, however, are strikingly alike.
“All five worlds are inclined at less than one degree to Pluto’s equator, indicating that there was no gravitational capture. While Pluto is reddish in color, the lack of volatiles is seen in all five moons, meaning that whatever impact likely created the moons also prevented these same moons from hanging onto the lightest elements and molecules. And finally, close-up studies of the four small moons — Styx, Nix, Kerberos, and Hydra — all indicate that these bodies coalesced out of multiple, smaller bodies that later became gravitationally bound.”
When the Hubble Space Telescope discovered additional moons of Pluto, beyond Charon, it was speculated that New Horizons might find more. After all, objects more than ten times as far away as Hydra, Pluto’s outermost moon, would still be in stable orbits. Yet, with five inner moons and nothing beyond, not even diffuse rings, the spacecraft came up empty. This isn’t a disappointment, though! Instead, New Horizons’ detailed observations of all five of Pluto’s moons point towards a tremendous picture: that the entire Plutonian system owes its origin to a massive, ancient collision. The debris kicked up created the five grey moons, in stark contrast to the reddish color of Pluto, in a near-perfect 1:3:4:5:6 resonance.